DAVENPORT, IA - SEPTEMBER 25: Republican presidential candidate Carly Fiorina addresses the Quad Cities New Ideas Forum at St. Ambrose University on September 25, 2015 in Davenport, Iowa. Fiorina is currently polling in second place behind Donald Trump for the Republican nomination. (Photo by Scott Olson/Getty Images)

MYRTLE BEACH, SC - SEPTEMBER 22: Republican presidential candidate Carly Fiorina speaks to voters during a town hall meeting at the Ocean Reef Convention Center September 22, 2015 in Myrtle Beach, South Carolina. Fiorina is a former Chief Executive Officer of Hewlett-Packard and currently chairs the non-profit philanthropic organization Good360. (Photo by Sean Rayford/Getty Images)

GREENVILLE, SC - SEPTEMBER 18: Republican presidential candidate Carly Fiorina speaks to voters at the Heritage Action Presidential Candidate Forum September 18, 2015 in Greenville, South Carolina. Ten republican candidates were each given 25 minutes to talk to the crowd at the Bons Secours Wellness Arena in the upstate of South Carolina. (Photo by Sean Rayford/Getty Images)

DERRY, NH - SEPTEMBER 6: Republican presidential candidate Carly Fiorina campaigns in New Hampshire over Labour Day weekend and meets with locals at MaryAnn's Diner on September 6, 2015 in Derry, New Hampshire. (Photos by Charles Ommanney/The Washington Post via Getty Images)

LITTLETON, NEW HAMPSHIRE - AUGUST 20: Carly Fiorina meets New Hampshire voters at a Spaghetti Dinner in the north country of Littleton, New Hampshire on Thursday, August 20, 2015. (Photo by Melina Mara/The Washington Post via Getty Images)

The two Republican rivals have feuded on and off since early in the campaign season. Most notably, Trump appeared to take a shot at Fiorina's looks in a September interview with Rolling Stone.

"Look at that face!" Trump was quoted as saying. "Would anyone vote for that? Can you imagine that, the face of our next president. I mean, she's a woman, and I'm not supposed to say bad things, but really, folks, come on. Are we serious?"

Fiorina was pushed back into Thursday's undercard debate — which is also being held in Charleston — because of slumping poll numbers. Trump, as he has been for each of the first five debates, remains front and center at the main event.